1. Field of the Invention
The present invention relates to conveying of a recording medium in an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a photocopier, a facsimile device, or a printing machine, toner images are formed on photoreceptors used as image bearing members to be electrostatically transferred to a recording medium such as a paper, an overhead projector (OHP) sheet, or a tracing paper and the transferred toner images are fixed and then a copied output is obtained.
The copied output can include a single color image or a full color image. As one of structures to form the image of the colors, a tandem type is known in which even when a full color image is output, the same level of output speed can be achieved as that of when a monochrome image is output.
In the tandem type image forming apparatus, each of image forming units corresponds to one of colors and includes the photoreceptor and a part of a device that performs an image forming process for the photoreceptor. The image forming units are arranged side by side. Images are sequentially transferred to a belt rotating around the image forming units or sequentially transferred to a recording medium such as a sheet fed by another belt and overlapped. Thus, a color image is obtained.
In the tandem type image forming apparatus, either of an intermediate transfer system and a direct transfer system can be used. In the intermediate transfer system, images formed on the photoreceptors are sequentially temporarily transferred to an intermediate transfer belt used as an intermediate transfer body by a primary transfer device, and then the images on the intermediate transfer belt are secondarily transferred to the recording medium, such as the sheet or the transfer paper, at one time by a secondary transfer device. On the other hand, in the direct transfer system, images formed on the photoreceptors are sequentially and directly transferred by a transfer device to the recording medium, such as the transfer paper, fed by a feeding belt.
In the tandem type image forming apparatus, a transfer roller is arranged on an inner side of the intermediate transfer belt or the feeding belt as a transfer bias applying unit that applies a transfer bias when the images are transferred. The transfer roller applies the transfer bias to the belt in a state where the transfer roller contacts with the inner side of the belt.
In the primary transfer, the images of each color are sequentially transferred to the intermediate transfer belt in an overlapping manner to obtain a multi-color image, and the multi-color toner image is transferred to the transfer paper at one time in the secondary transfer.
The recording medium (hereinafter, “transfer paper”) fed to a secondary transfer position where the secondary transfer is performed is held to be fed at a position of a transfer nip portion, a position of a registration roller, and a position of a feeding roller respectively. The transfer nip portion is formed with a secondary transfer roller and a backup roller that presses the intermediate transfer belt toward the secondary transfer roller. The registration roller sets a registration timing of the transfer paper fed to the transfer nip portion. The feeding roller is provided in a transfer path of the transfer paper fed from a paper feeding device.
When the transfer paper is fed to the transfer nip portion, the orientation of the transfer paper that is to enter into the transfer nip portion can be controlled by an entrance guide member and the like. A related technology has been disclosed, for example, in Japanese Patent Application Laid-open No. 2000-229748 and Japanese Patent Application Laid-open No. 2002-156802.
As described in Japanese Patent Application Laid-open No. 2000-229748 and Japanese Patent Application Laid-open No. 2002-156802, there is a problem that toner scattering occurs due to a phenomenon called “premature transfer” in which an electric discharge occurs before the transfer paper arrives at the nip portion in an entrance of the transfer nip portion. Toner scattering refers to a phenomenon that transfer toners borne on the intermediate transfer belt are scattered to be unexpectedly transferred to the transfer paper due to an occurrence of a premature transfer electric discharge between the transfer paper and the intermediate transfer belt at the location upstream from the transfer nip portion when the transfer paper makes an improper angle with respect to the intermediate transfer belt.
Especially, when a rear end portion of the transfer paper passes through the entrance guide member and the transfer paper returns to its original shape by the flexural rigidity determined by its thickness, the rear end portion comes into contact with the transfer belt unstably and in a vibrating manner. This can create a disorder of the toner images.
To prevent this problem, the entrance guide member needs to be arranged such that a contact between the transfer belt and the transfer paper does not occur. An end portion of the entrance guide member near the transfer nip is frequently arranged closer to the transfer nip portion.
However, if this configuration is adopted, at the instant when the rear end portion of the transfer paper comes loose from the entrance guide member after the direction of the transfer paper is controlled by the entrance guide member and the transfer paper returns to its original shape and moves toward the intermediate transfer belt due to the thickness of the transfer paper that determines its flexural rigidity, the transfer paper can hit the intermediate transfer belt. When the transfer paper hits the intermediate transfer belt, the impact is large and it may cause the disorder of the images to become large.